The invention is related to the field of cable television systems and more specifically with amplification of multi-carrier video signals in cable television systems.
In a cable television system (CATV), television programs are provided at a central head-end. The programs are distributed from the head-end through branching tree-like networks of optical fibers to a multitude of hybrid fiber cable nodes (HFCNs) in respective local communities. Then further distributed from the HFCNs through branching tree-like networks of coaxial cables to customer interface units (CIUs), also called cable terminations.
Currently, many of these systems are beginning to provide additional communication services such as telephone services and computer networking services (e.g. internet connection) through the cable television system. Telephone and computer networking services require bi-directional communication in the cable television system. Forward data signals for these additional services are transmitted in a manner similar to television signals, as described above, and return data signals are transmitted through the same path in the reverse direction. That is, return signals are collected from the CIUs through the branching coaxial cable networks to the HFCNs, back through the HFCNs, and back through the branching optical fiber network to the head-end.
At the head-end, a multitude of electronic forward information signals for broadcast television and additional services (telephone and computer communications) are used to modulate respective carrier signals of different frequencies. The modulated carrier signals are combined into an electronic multi-carrier forward signal that is used to modulate a forward laser beam to produce an optical forward signal carried by the forward laser beam. The modulated laser beam, carrying the optical forward signal, is transmitted through the optical fiber network to a multitude of the HFCNs. At each local node an optical detector coverts the optical forward signal back into an electronic forward signal. The reconverted electronic forward signal is transmitted from the HFCNs through the coaxial cable network to CIUs at homes and businesses of customers.
At the cable termination, telephone and computer equipment of the customer, are connected to the CIUs. The customer""s equipment produce electronic return signals that are transmitted by the CIUs into the coaxial cable network. The return signals are multi-carrier modulated signals similar to the forward signals. The return signals travel back through the tree-like coaxial cable network to the HFCNs. In the HFCNs, the return signals are separated from the forward signals by diplex filters. The separated return signals are used to modulate a return laser beam to produce a multi-carrier optical return signal carried by the return laser beam. The optical return signal is transmitted back through the tree-like optical fiber network to the head-end where the optical return signals are converted back into electronic return signals by an optical detector for the return signals. The electronic return signals are demodulated and used for telephone and computer communications.
Requirements for signal to noise ratio (S/N) at the cable termination together with limits on the allowed optical power, limit the length of one-directional optical transmission of analog television signal to around 100 km. In the coaxial cable network, line amplifiers are required at intervals of approximately 300 to 350 meters in order to maintain the amplitude of the high frequency electronic signals. The line amplifiers in the coaxial cable network produce distortions that result in additional noise that further limits the length of signal transmission.
In bi-directional transmission, the introduction of return light beams in the optical fiber network results in crosstalk as additional noise that further reduces the range of cable broadcasting. The line amplifiers must be bi-directional and both the forward and return amplifiers produce distortions that result in increased noise in both the forward and return directions which further limits transmission distance.
An important part of the distortion caused by power amplifiers is the composite triple beat (third order) distortion. In addition to the two amplifiers in each bi-directional line amplifier, the optical transmitters, optical receivers, and CIUs each include a power amplifier. The distortions are cumulative as the signal passes through a multitude of power amplifiers from the source of the signal to the CIUs, and the distortions from return signal amplification in the line amplifiers also adds to the distortion of the forward signals. The result is that signal transmission in bi-directional systems is even more limited by noise than in previous one-directional systems.
Those skilled in the art are directed to the following citations. U.S. Pat. No. 4,947,386 to Preschutti discloses a broadband network with a bi-directional amplifier. U.S. Pat. No. 5,343,158 to Gris discloses another bi-directional amplifier. U.S. Pat. No. 5,519,434 in FIG. 2 discloses an all pass filter.
The above references are hereby incorporated herein in whole by reference.
A broadband communication system includes multi-stage power amplifier systems for amplifying the power of radio-frequency (RF) communication signals. Each stage of the amplifier systems result in composite triple beat (CTB) distortion, and if the phase of the CTB distortions are approximately the same (i.e. are in-phase), then the amplitudes of the distortions are added (i.e. xe2x80x9c20 dBxe2x80x9d rule). The amplifier system of the invention includes one or more phase filters positioned in series between the power amplifier stages. The phase filters are adapted to shift the phase of the communication signals, so that, the phase of CTB distortions, resulting from the amplification of the communication signals in the amplifier stages between the phase filters, are substantially different (i.e. out-of-phase). Thus, only the power of the CTB distortions are added (i.e. xe2x80x9c10 dBxe2x80x9d rule).
Preferably, the shift in-phase response of the phase filters, over the frequency band to be amplified by the power amplifier, is at least 30 degrees over at least 15% of the band. Preferably, the multi-stage power amplifier is provided as an amplifier unit on a plug-in card to allow the invention to be easily implemented on existing equipment such as line amplifiers.
The invention includes a particular phase filter that allows the phase of a communication signal to be reliably shifted by amounts controlled by selecting the properties of the components of the phase filter.
The invention also includes a bi-directional line amplifier that uses the multi-stage phase shifted power amplifier of the invention. The invention is especially useful for such line amplifiers because several such amplifiers are often required in series along the coaxial cable networks of a broadband network system. It is an important aspect of the invention that phase filters be used to prevent the amplitudes of CTB distortions of the series of line amplifiers from being combined additively.
The invention also includes an optical transmitter using the multi-stage phase shifted power amplifier of the invention. The power amplifier is required to provide the correct power for modulating the laser beam. These optical transmitters are used in the cable television system to transmit the communication signals through optical fibers in a forward direction from the head-end through hybrid fiber cable nodes (HFCNs) and to transmit return signals back through the optical fibers from the HFCNs to the head-end. The CTB distortion resulting from the power amplification in the optical transmitters accumulates with the CTB distortions of the line amplifiers to produce noise in the system.
The invention also includes an optical receiver using the multi-stage phase shifted power amplifier of the invention. These receivers usually include a preamplifier to amplify the signal for post-processing the signal and after post-processing the signal is amplified for further distribution. These optical receivers are used to receive the forward communication signals from the optical fibers at the HFCNs and provide the amplified signal into the coaxial cable networks. Also, these optical receivers are used to receive the return communication signals from the optical fibers at the head-end. The CTB distortion resulting from the power amplification in the optical receivers accumulates with the CTB distortions of the line amplifiers and power amplifiers in the optical transmitters to produce noise in the system.
The invention also includes the head-end, an optical hub, and HFCNs that use the transmitters and receivers of the invention that utilize the multi-stage phase shifted power amplifier of the invention.
The invention reduces the accumulated amplitude of the different CTB distortions produced by power amplifiers in several different types of equipment in the communications link of a cable television system. Both, the CTB distortions in the forward signal between the head-end and the CIUs are reduced and the CTB distortions in the return signals from the CIUs to the head-end are reduced.
Those skilled in the art will understand the invention and additional objects and advantages of the invention by studying the description of preferred embodiments below with reference to the following drawings that illustrate the features of the appended claims: